U.S. Pat. No. 5,260,248 discloses catalysts comprising oxides of manganese, copper and lanthanum having composition, expressed as MnO, CuO and La2O3, of 60-75 wt % MnO, 15-23 wt % CuO and 10-18 wt % La2O3. The thermal stability of the catalyst is not higher than 700° C.: in any case, higher than that of perovskite type catalysts, which is of about 650° C.
A typical perovskite oxidation catalyst has formula LaMn 0.5 Cu 0.5 O3 (U.S. Pat. No. 3,914,389).
The catalysts of U.S. Pat. No. 5,260,248 are prepared by impregnation of alumina with aqueous solutions of nitrates of Mn, Cu and La, followed by drying and calcination at 400°-700° C.
EP 1197259 A1 describes mixed oxides catalysts comprising oxides of Mn, Cu and rare earth metals, having composition, expressed as MnO, CuO and oxides of rare earth metals at the lowest valence state, of 14-88 wt % MnO, 10-66 wt % CuO and 2-20 wt % La2O3. The preferred composition is 44-55 wt % MnO, 33-43 wt % CuO and 10-13 wt % La2O3.
The preparation of the catalysts of EP 1197259 A1, when supported on a carrier, require in the industrial scale preparation two impregnation steps made after the first impregnation with the lanthanum nitrate, each of which carried out after drying and calcination at 600° C.
The above multistep operation is expensive being labour intensive.
US 2004/0151647 A1 discloses mixed oxides catalysts of composition expressed as wt % metal: 15-30% Cu, 55-77% Mn and 7.5-10% Ce wherein Ce can be substituted with Pr or 0.2-5 wt % La.
It is an object of this present invention to provide mixed oxides catalysts suitable for the selective oxidation of VOC compounds to CO2, as well as for the decomposition of nitrogen protoxide to nitrogen and oxygen and the combustion of CO, H2 and CH4 off gases from the anode of fuel cells, endowed of high thermal stability and high activity even after ageing at high temperatures, and not requiring in the industrial preparation more than one step impregnation after the impregnation with the lanthanum nitrate solution.
The above and other objects will be apparent from the following description of the invention.